This invention relates in general to a projection television system utilizing a cathode ray tube in association with an optical lens, and is particularly concerned with means for enhancing the contrast of an image projected by the tube-lens arrangement.
A color projection television system of the type herein considered is disclosed and claimed in U.S. Pat. No. 4,376,949 which issued to William A. Rowe, and which is assigned to the assignee of the present invention. The Rowe patent is addressed to a system which employs three different monochrome picture tubes, which independently develops one of each of the three primary colors. The images formed by these tubes are projected through a lens arrangement to the end that a color image is displayed on a viewing screen.
U.S. Pat. No. 4,388,642 which issued to James J. Reno et al., and which is assigned to the assignee of the subject invention, albeit being addressed to a heat sink means for a projection CRT, depicts the manner in which a projection CRT is arranged to cooperate with an optical lens in a projection television system.
U.S. Pat. No. 4,405,949 which issued to Richard L. Hockenbrock et al, and which is assigned to the assignee of the subject invention, describes and claims a liquid cooled projection CRT. It is a projection CRT of this construction that the subject invention finds particular application for effecting a direct optical coupling between an image displaying the surface of that tube and an associated projection lens element.
U.S. Pat. No. 4,150,320 which issued to Robert B. Mitchell et al., is addressed to an integral lens cathode ray tube system comprising a tube having a spherically shaped viewing portion 9 and a glass panel 13 which is affixed to the viewing portion by an optically matched resin 11 to form an integrated optical interface providing a double concave lens configuration. It is suggested that the disclosed arrangement minimizes defocusing of an image from a spherically shaped CRT when directed upon a flat surface, as is the practice in a projection television system.
While the disclosure is silent in the matter, it would appear that if the CRT in this integrated arrangement should fail, in addition to the loss of the tube, the resin interface as well as glass panel 13, the second half of the double concave lens, would also have to be discarded.
Other known integrated projection tube arrangements utilizing a resin interface also have the aforementioned shortcoming, that is, failure of any one component results in loss of the entire package.
Aside from the foregoing, a particular problem characteristic in projection TV systems in which the developed image is air-coupled from the CRT face panel to the lens assembly is veiling glare. This glare is attributable to reflection of light rays from the surface of the face panel panel back to the inside wall of the panel immediately beneath the electroluminescent screen. The initial reflection is caused by virtue of the difference in index of refraction as between the glass material in the face panel and the air outside the panel wall.
In any event, it will be shown that when the aforementioned air gap is eliminated in accordance with the invention, veiling glare is substantially reduced and the contrast ratio in the image projected on the viewing screen is significantly improved.